This invention relates generally to improvements in liquid infusion devices of the type used for controlled administration of medication to a patient. More specifically this invention relates to an improved medication infusion device having a space-efficient drive system.
Infusion devices are well known in the art for use in delivering medication, such as insulin to a patient. U.S. Pat. No. 5,637,095 titled ‘Medication infusion pump with flexible drive plunger’ includes a compact drive motor mechanically coupled by a flexible drive member to a sliding piston for delivering medication to a patient. The flexible drive member extends through a space-efficient curved path, and comprises a length of spring tape formed from spring steel to have a curved cross sectional shape when oriented in linear configuration. The spring tape is wrapped or coiled onto a take-up spool within the pump housing. Drive means may be a lead screw nut carried on an elongated lead screw, with the drive motor providing a rotary output for driving the lead screw in a manner to advance the lead screw nut along the lead screw. Linear displacement of the lead screw nut translates the spring tape along its curved path. Or preferably a capstan roller and associated pinch roller engage and advance the spring tape under control of the pump drive motor, with a length of the spring tape loosely suspended and guidably-received within a curved path at one side of the drive means. In operation, when a medication-containing barrel is loaded into the pump housing, the pinch roller is retracted relative to the capstan roller to permit the tape to be inserted into or removed from the space between these rollers.
U.S. Pat. No. 6,537,251 titled ‘Medication delivery device with bended piston rod’ describes a flexible piston rod consisting of two separate tape-shaped bodies, joined together at one or more points, optionally describing an ‘eye-shaped’ path when viewed in a transversal cross-section in a relaxed state. Movement of the piston rod is activated by an electromotor whose rotational movement is transferred to a linear displacement of the piston rod by suitable driving means, comprising a driving wheel. Said driving wheel including regularly spaced protrusions that interact with corresponding receiving members on the flexible piston rod (optionally isolated through-holes or slots) to displace the piston rod. In one embodiment, the piston rod is bent to make a 180 degrees U-turn over a first guiding wheel, and a second guiding wheel ensures proper contact between the piston rod and the driving wheel. In the longitudinal direction, the smallest diameter of the wheel is limited by the smallest diameter around which the rod may be elastically bent.
U.S. Pat. No. 5,957,889 titled ‘Displacement system for controlled infusion of a liquid’ describes a liquid displacement system having a piston rod as a flexible incompressible construction which is guided by a piston rod guide behind the rear end of the cartridge deflected away from the axis of the cartridge, preferably 180 degrees. The guide includes a track elaborated to the very shape which the curved part of the piston rod will spontaneously adopt when it's end portions are kept parallel, enabling the length of the device to be reduced to correspond to about the length of the cartridge and the deflecting piston rod guide. The flexible rod may be a flexible helix with narrowly adjacent turns of windings, and a coiling ratio within certain limits. The windings of the helix present an external thread which may be engaged by an internally threaded nut element which, when rotated, will drive the piston rod into the cartridge in conjunction with a presser foot acting on the free end of the piston rod.
The aforementioned, and other prior art presents many problems to be overcome. For example, locating the drive system relatively far from the plunger typically requires a thick piston rod that may require a high power consuming motor, possibly introducing inaccuracies in the displacement of the piston. Also, systems including several components such as drive wheels, take-up spools and additional supports or guides for the path of the tape whilst outside of the cartridge, lead to complex systems that may be more difficult to use and manufacture, whilst potentially introducing inaccuracies in the regulation of liquid infusion due to mechanical friction or component wear for example. Pumps including a cartridge typically require at least one dimension to be greater than twice the length of the cartridge in order to provide enough space for a piston rod to be fully retracted when a new, full cartridge is present. The invention disclosed herein provides a space-efficient drive system for a liquid infusion device.